The present invention relates to a light scattering type liquid crystal display (LCD) and, more particularly, to a reflection type LCD made up of display elements each having a liquid crystal dispersion layer in which liquid crystals are dispersed in a tridimensional microstructure implemented by a polymer or similar resin.
One of conventional LCD display systems uses a pair of polarizers to implement the birefringence and optical rotary power particular to liquid crystals. This type of display system is operable in a TN (Twisted Nematic) mode or an STN (Super Twisted Nematic) mode. However, the problem with this type of display system is that the display is dark due to the short sharpness in electrooptic characteristic and the presence of two polarizers. When a dichroic dye is added to liquid crystals for a guest-host (GH) effect, a single polarizer suffices and renders the display light to some degree. This approach, however, reduces the contrast to a critical degree. Particularly, in the event of color display, it is impossible to use the TN or STN mode GH type configuration since color display requires a high contrast. It follows that a color LCD implemented by any one of birefringence modes, including a DAP (or VAN) mode and an ECB mode using horizontally oriented cells, has to be provided with an extremely bright back-light. Although LCD elements themselves consume only far smaller power than the other display elements, bright illuminating means critically degrade the power saving feature. Moreover, despite that the LCD elements are originally thin and flat, the illuminating means increases the thickness of the assembly. Even monochromatic display appears dark when the back-light is not used.
An active matrix type LCD has TFTs or MIMs each being assigned to one pixel and can compensate for the short sharpness particular to the TN mode. Therefore, this type of LCD provides far more light display than the above-stated simple matrix type LCD. However, even the active matrix type LCD uses two polarizes and, therefore, cannot implement sufficiently light display without using a back-light. Further, using two polarizers, which are expensive, is undesirable from the cost standpoint. Bonding the polarizers to the LCD elements without introducing air bubbles or creases is difficult, resulting in a substantial decrease in yield. Generally, the polarizers are fabricated by introducing iodine into elongated polyvinyl alcohol (PVA) or by elongating iodine-containing PVA. The polarizers are, therefore, short of resistivity to heat and moisture. It may safely be said that the reliability of the state of the art LCD elements is determined by the polarizers.
A light scattering system using light scattering by liquid crystals in place of the polarizers is a recent achievement and known as a dynamic scattering mode or a phase transition mode. For example, Japanese Patent Laid-Open Publication (Kokai) No. 58-501631 proposes a polymer dispersion type LCD element in which droplets of liquid crystals are dispersed in a polymer matrix. This kind of element is hardly susceptible to the thickness of the liquid crystal layer and implements a broad display area. In addition, such an element does not need any polarizer and, therefore, eliminates light losses which would render the display dark. On the other hand, Japanese Patent Laid-Open Publication No. 1-198725 discloses a polymer network type LCD element in which liquid crystals are dispersed in a tridimensional mesh structure constituted by a photosetting resin. This type of element requires only a low drive voltage and exhibits high sharpness while achieving the advantages stated above in relation to the polymer dispersion type element.
Since the above-described types of LCD elements modulate light and display data due to the light scattering ability of the liquid crystal layer, it is extremely difficult to implement an element for direct watching without relying on the GH configuration. For this reason, such LCDs have heretofore been developed for the application to projection type displays. However, when applied to a projection type display, the element has to be provided with an intense lamp and optics for projection which would increase the thickness more than the back-light scheme. Moreover, the GH type configuration often lacks reliability due to impurities contained in the dichroic dye and products of optical decomposition. In addition, when a polymer and liquid crystal dispersion structure is produced by use of a photosetting resin, it is likely that the dye obstructs the photosetting reaction. This limits the material available for the dispersion structure.
Generally, when the light scattering type LCD element is applied to a reflection type display for direct watching, it is desirable that the background of the element is black. Then, the element will appear black in a transparent state or appear white in a scattering state due to rearward scattering thereof. However, this kind of element has a problem that the intensity of light scattered rearward is not high enough to render sufficient whiteness. Although this problem may be eliminated if the thickness of the liquid crystal layer is reduced, this approach brings about another problem that the required drive voltage increases. Particularly, the polymer dispersion type LCD element originally needs a higher operation voltage than the other scattering type elements. Japanese Patent Publication No. 45-21729 teaches a system in which a mirror surface is provided on the rear of a liquid crystal cell, and means having a black surface is provided on the top of the element to prevent regular reflections of extraneous light from being directly incident to the viewer's eye. In this conventional system, when a given pixel is in a scattering state, scattered light derived from light incident in a direction other than the regular reflection direction is incident to the viewer's eye, providing white display; when the pixel is in a transparent state, the light does not reach the viewer, providing black display. This kind of scheme, however, prevents the LCD from having a flat configuration and causes the lightness of the white portion to be short.